1. Field of the Invention
The present invention relates to a flat panel display device and more particularly, to an electroluminescent display (ELD) device and method of fabricating the same.
2. Discussion of the Related Art
Generally, organic electroluminescent display (ELD) devices have electron-input electrodes that are commonly referred to as cathode electrodes and hole-input electrodes that are commonly referred to as anode electrodes. The electrons and the holes are input into an light-emitting layer from the cathode and anode electrodes, respectively, wherein the electron and hole together form an exciton. The organic electroluminescent display (ELD) device emits light when the exciton is reduced from an excited state level to a ground state level. Accordingly, since organic electroluminescent display (ELD) devices do not require additional light sources, both volume and weight of the organic electroluminescent display (ELD) devices may be reduced. In addition, the organic electroluminescent display (ELD) devices are advantageous because of their low power consumption, high luminance, fast response time, and low weight. Presently, the organic electro luminescent display (ELD) devices are commonly implemented in mobile telecommunication terminals, car navigation systems (CNSs), personal digital assistants (PDAs), camcorders, and palm computers. In addition, since manufacturing processes for the organic electroluminescent display (ELD) devices are simple, manufacturing costs can be reduced as compared to liquid crystal display (LCD) devices. The organic electroluminescent display (ELD) devices may be classified into passive matrix-type and active matrix-type. Though the passive matrix-type organic electroluminescent display (ELD) devices have simple structures and manufacturing processes are simple, they require high power consumption and are not suitable for large-sized display devices. In addition, aperture ratios decrease as the number of electro lines increase. On the other hand, the active matrix-type organic electro luminescent display (ELD) devices have high light-emitting efficiency and high image display quality.
FIG. 1 is cross sectional view of an organic electroluminescent display (ELD) device according to the related art. In FIG. 1, the organic electro luminescent display (ELD) device 10 has a transparent first substrate 12, a thin film transistor array part 14, a first electrode 16, an organic light-emitting layer 18, and a second electrode 20, wherein the thin film transistor array part 14 is formed on the transparent first substrate 12. The first electrode 16, organic light-emitting layer 18, and second electrode 20 are formed over the thin film transistor array part 14. The light-emitting layer 18 displays red (R), green (G), and blue (B) colored light, and it is commonly formed by patterning organic material separately for each pixel for the R (red), G (green) and B (blue) colored light. A second substrate 28 has a moisture absorbent desiccant 22. The organic electroluminescent display (ELD) device 10 is completed by bonding the first and second substrates 12 and 28 together by disposing a sealant 26 between the first and second substrates 12 and 28. The moisture absorbent desiccant 22 removes moisture and oxygen that may be infiltrated into an interior of the organic electroluminescent display (ELD) device 10. The moisture absorbent desiccant 22 is formed by etching away a portion of the second substrate 28, filling the etched portion of the second substrate 28 with moisture absorbent desiccant material, and fixing the moisture absorbent desiccant material with tape 25.
FIG. 2 is a plan view of a thin film transistor array pixel part of an organic electroluminescent display (ELD) device according to the related art. In FIG. 3, the pixel includes a switching element TS, a driving element TD, and a storage capacitor CST at every pixel region “P” defined on a substrate 12. The switching element TS and the driving element TD may be formed with combinations of more than two thin film transistors. The substrate 12 is formed of a transparent material, such as glass and plastic. A gate line 32 is formed along a first direction, and a data line 34 is formed along a second direction perpendicular to the first direction. The data line 34 crosses the gate line perpendicularly with an insulating layer between the gate and data lines 32 and 34. A power line 35 is formed along the second direction, and is spaced apart from the data line 34. The thin film transistor used for the switching element TS has a gate electrode 36, an active layer 40, a source electrode 46, and a drain electrode 50, and the thin film transistor for the driving element TD has a gate electrode 38, an active layer 42, a source electrode 48, and a drain electrode 52. The gate electrode 36 of the switching element TS is electrically connected to the gate line 32, and the source electrode 46 of the switching element TS is electrically connected to the data line 34. The drain electrode 50 of the switching element TS is electrically connected to the gate electrode 38 of the driving element TD through a contact hole 54, and the source electrode 48 of the driving element TD is electrically connected to the power line 35 through a contact hole 56. The drain electrode 52 of the driving element TD is electrically connected to a first electrode 16 within the pixel region “P,” wherein the power line 35 and a first capacitor electrode 15 that is formed of polycrystalline silicon layer form a storage capacitor CST.
FIG. 3 is a cross sectional view along III—III of FIG. 2 according to the related art. In FIG. 3, only cross sectional views of the driving element and the light-emitting part are illustrated. As shown in FIG. 3, the organic electro luminescent display (ELD) device has the driving element, i.e., a thin film transistor TD, a first electrode 16, a light-emitting layer 18, and a second electrode 20. The driving thin film transistor TD has a gate electrode 38, an active layer 42, a source electrode 56, and a drain electrode 52. The first electrode 16 is formed over the driving thin film transistor TD and is connected to the drain electrode 52 of the driving thin film transistor TD with an insulating layer between the first electrode 16 and the driving thin film transistor TD. The light-emitting layer 18 is formed on the first electrode 16 for emitting light of a particular color wavelength, and the second electrode 20 is formed on the light-emitting layer 18. A storage capacitor CST (in FIG. 2) is connected in parallel to the driving thin film transistor TD, and includes first and second capacitor electrodes 35 and 15. The source electrode 56 of the driving thin film transistor TD contacts the second capacitor electrode 35, i.e., a power line, and the first capacitor electrode 15 is formed of polycrystalline silicon material under the second capacitor electrode 35. The second electrode 20 is formed on the substrate 12 on which the driving element TD, the storage capacitor CST, and the organic light-emitting layer 18 are formed. Accordingly, the organic electroluminescent display (ELD) device is a bottom emission-type device, wherein the light-emitting layer emits the light downward of the substrate.
The organic electroluminescent display (ELD) device is commonly manufactured by forming the thin film transistor array part and the light-emitting part on a same substrate, and then bonding the substrate to an encapsulating structure. If the thin film transistor array part and the light-emitting part are formed on the same substrate, then a yield of a panel having the thin film transistor array portion and the light-emitting part is dependent upon the product of the individual yields of the thin film transistor array part and the light-emitting part. However, the yield of the panel is greatly affected by the yield of the organic light-emitting layer. Accordingly, if an inferior organic light-emitting layer that is usually formed of a thin film having a thickness of 1000 Å has a defect due to impurities and contaminants, the panel is classified as a inferior panel. This leads to wasted production costs and material, thereby decreasing the yield of the panel.
The bottom emission-type organic electroluminescent display (ELD) devices are advantageous for their high image stability and variable fabrication processing. However, the bottom emission-type organic electroluminescent display (ELD) devices are not adequate for implementation in devices that require high resolution due to limitations of increased aperture ratio. In addition, since top emission-type organic electroluminescent display (ELD) devices emit light upward of the substrate, the light can be emitted without influencing the thin film transistor array part that is positioned under the light-emitting layer. Accordingly, design of the thin film transistor may be simplified. In addition, the aperture ratio can be increased, thereby increasing operational life span of the organic electroluminescent display (ELD) device. However, since a cathode is commonly formed over the light-emitting layer in the top emission-type organic electroluminescent display (ELD) devices, material selection and light transmittance are limited such that light transmission efficiency is lowered. If a thin film-type passivation layer is formed to prevent a reduction of the light transmittance, the thin film passivation layer may fail to prevent infiltration of exterior air into the device.